Fossil crust‐to‐mantle transition, Val Malenco (Italian Alps)

Abstract

An exhumed, undisturbed fossil lower crust to upper mantle section is preserved in Val Malenco, Italian Alps, and is now exposed along the boundary between Penninic and Austroalpine nappes. Lower‐crustal metapelitic rocks are welded to upper‐mantle ultramafic rocks by a mid‐Permian gabbro intrusion. The tmderplating of gabbro caused granulite metamorphism and partial melting of the metapelites. In the crust‐to‐mantle transition zone of at least 1 km thickness, gabbros, large xenoliths of restitic metapelites and ultramafic rocks occur, with densities of 2.95–3.14, 3.25 and 3.27 g/cm 3 , respectively. The seismic Moho therefore did not coincide with the boundary between peridotites and crustal rocks but was situated above the upper limit of the peridotitic mantle. The whole complex underwent cooling with only moderate decompression within the kyanite field. This process started at 1 GPa and ∼800°C and ended at 0.85 GPa and 600°C and is interpreted as thermal relaxation after the gabbro intrusion. Later, during Jurassic rifting, the crust‐to‐mantle section was exhumed at the Adria margin of the Tethys ocean.